Silicon tetrafluoride is an important chemical intermediate, useful for the production of valuable products, such as pure silica, silanes, pure silicon for solar cells, silicon nitride for ceramic products and fluorinated carbon-silicon polymers for materials for architectural uses. Other uses of silicon tetrafluoride include: treating dried concrete parts in order to provide a considerable improvement of their waterproofness and resistance to corrosion and abrasion; increasing the hydrophobic character of crystalline molecular sieves for producing orthosilicic acid esters; and as an etching medium for materials containing silicon in the semiconductor industry.
Known methods for producing silicon tetrafluoride, along with hydrogen fluoride, include reacting sulfuric acid with fluorspar, forming calcium sulfate as by-product. The reaction is endothermic and heat must be externally provided. Methods have been devised to improve heat transfer characteristics, yield and purity. Fluorosilicic acid, from phosphoric acid production, may also be used as feedstock to produce hydrofluoric acid and silicon tetrafluoride. In general, a stream of concentrated sulfuric acid, or oleum, and a concentrated aqueous solution of fluorosilicic acid are fed to a stirred reactor, producing hydrofluoric acid and silicon tetrafluoride in the form of a gas stream, which is washed by concentrated sulfuric acid. Also known is a process in which a stream of aqueous fluorosilicic acid is fed to an intermediate point between the head and the bottom of a vertical tower, and a stream of concentrated sulfuric acid is fed near the head of the tower. From the head of the tower, an overhead gas stream containing silicon tetrafluoride is recovered, and from the bottom of the tower a stream of diluted sulfuric acid is recovered. Yields of 95.4% to 98.5% are achieved, with a content of hydrofluoric acid in the silicon tetrafluoride, being lower than 0.1% by volume.
Other processes for manufacturing silicon tetrafluoride are based on elemental silicon. Elemental silicon and hydrogen fluoride are reacted at temperatures of about 250° C. or higher. The reaction may be conducted such that the gas product contains at least 0.02 volume % of the unreacted hydrogen fluoride. The process may be improved by contacting the gas product with elemental nickel at a temperature of 600° C. or higher.
A characteristic common to all these processes is constituted by the low yields of conversion of raw materials into silicon tetrafluoride and undesirable by-products. The impurities, in particular, compounds of fluorine, boron, phosphorus and arsenic elements which remain in the silicon tetrafluoride.
In US20100189621 entitled Improved Process of Silicon Tetrafluoride Gas Synthesis, the present inventors disclosed and claimed a process for producing silicon tetrafluoride with high conversion and purity from sources of fluoride with limited environmental impact. The disclosure of such application is incorporated herein in its entirety by reference. Specifically, that process produced silicon tetraflouride (SiF4), from metal fluorides, silica and sulfuric acid.
The kiln discharge, which is a byproduct of the production of SiF4 according to the process described in US20100189621, is a mixture of sulfates of aluminum and sodium. In addition, this byproduct may further include sulfates of other metals, including sulfates of calcium, titanium and iron. For example, the following complex sulfates may be found in the kiln discharge: NaAl(SO4)2xH20; Ca6Al(SO4)3(OH)2xH2O; NaaCabAlc(SO4)d.xH20; Na2(SO4)2xH20; Al2(SO4)3xH2O; and Ca(SO4)xH20, where a, b, c and d are integers. The composition of the byproduct depends on the starting component used for silicon tetrafluoride manufacturing. Depending upon the composition of the byproduct, the kiln discharge mixture components may be present as separated, individual components or as a compound mixture. For example, sodium-aluminum-calcium sulfate may be present as a single complex compound or as individual metal sulfates depending on the discharge conditions and reactor operation. Small amounts of unconverted feed materials (including silica, sulfuric acid and fluorides) are also expected to be present in the kiln discharge depending upon the type of operation and feed. There is a need for a process to separate out these byproduct compounds into valuable commercially viable commodity products in an environmentally sound manner.